What is Sumitomo General Fault Code 21?

Fault Code 21 on Sumitomo excavators indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal coolant temperature reading being transmitted to the Electronic Control Module (ECM). This diagnostic trouble code triggers when the ECM detects either an open circuit, short circuit, or out-of-range signal from the coolant temperature sensor, which is critical for proper engine management and protection systems.

The coolant temperature sensor provides essential data that the ECM uses to adjust fuel injection timing, regulate engine warm-up cycles, and activate cooling fan operation. On Sumitomo General excavators, this sensor directly influences engine performance parameters and hydraulic oil temperature management. When this code appears, the ECM typically defaults to a predetermined temperature value, which can compromise fuel efficiency and increase wear on engine components, particularly in used machines where optimal operating temperatures are crucial for longevity.

Common Symptoms

• Engine warning light illuminated on the instrument panel, often accompanied by an audible alarm
• Hard starting or rough idle during cold engine conditions due to improper fuel mixture compensation
• Cooling fan runs continuously at high speed, even when engine temperature is normal
• Reduced engine power or derate mode activation as the ECM implements protective measures
• Black smoke from exhaust indicating rich fuel mixture from incorrect temperature compensation

Potential Causes

The most common causes of Code 21 on used Sumitomo excavators include:

• Faulty coolant temperature sensor due to internal element degradation (common failure point after 5,000+ operating hours)
• Damaged or corroded wiring harness connections at the sensor plug, especially where harness routes near hot engine components or sharp metal edges
• Broken or chafed wires in the sensor circuit caused by engine vibration and movement over time
• Corroded ECM connector pins affecting signal transmission, particularly in machines operated in high-moisture environments
• Low coolant level causing sensor exposure and erratic readings
• Defective ECM (rare, but possible in older machines with 10,000+ hours)

How to Troubleshoot and Fix Code 21

Step 1: Visual Inspection Begin by checking the coolant level in the radiator and expansion tank. Inspect the coolant temperature sensor (typically located on the engine block or cylinder head) and its wiring harness for obvious damage, corrosion, or oil contamination. On used excavators, pay special attention to harness routing where it may contact hot surfaces or moving components.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance at ambient temperature (typically 2,000-3,000 ohms at 68°F/20°C, but consult your service manual for exact specifications). Test the wiring harness for continuity between the sensor connector and ECM, and check for shorts to ground using the ohmmeter function.

Step 3: Signal Verification With the sensor reconnected and engine cold, use Sumitomo diagnostic software or a compatible scan tool to monitor live coolant temperature data. Compare the displayed temperature against an infrared thermometer reading. If readings diverge by more than 10°F, replace the sensor. Check connector pins for corrosion and apply dielectric grease during reassembly.

Step 4: Clear and Verify After repairs, clear the fault code and operate the machine through a complete warm-up cycle. Monitor for code recurrence and verify proper cooling fan operation.

---

Disclaimer: This guide provides general diagnostic information. Always consult your Sumitomo service manual for model-specific procedures and specifications. If you lack proper diagnostic tools or experience with electrical systems, seek assistance from a qualified heavy equipment technician to avoid misdiagnosis or damage to ECM components.